This invention relates to a raster scanner, and more particularly, to a raster scanner optical system which utilizes an indexed rotating polygon mirror to reduce a problem known as misregistration of different colors on a scan line.
Typically, a single color raster scanner system has a light source, such as a laser, which emits a coherent light beam. The light beam is collimated in both the fast scan or tangential plane and in the cross scan or sagittal plane by multiple optical elements. The collimated light beam in the cross scan plane is focussed at a point near a facet of a rotating polygon mirror by a cylindrical optical element while in the fast scan plane the light beam remains collimated when the beam strikes the facet of the rotating polygon mirror.
The rotating polygon mirror causes the reflected beam to revolve about an axis near the reflection point of the rotating polygon mirror. This reflected beam can be utilized to scan a document at the input end of an imaging system as a raster input scanner or can be used to impinge upon a photosensitive medium, such as a xerographic drum (photoreceptor), in the output mode as a raster output scanner.
The light beam is modulated in accordance with the pixel information either at the laser diode (if the light source is laser diode) or prior to the rotating polygon mirror. In a raster output scanner, prior to scanning the modulated light beam, the surface of the photoreceptor is uniformly charged. Scanning the light beam across the surface of the photoreceptor, causes the electrostatic charge on the surface of the photoreceptor to selectively dissipate. The selection of the charge dissipation is based on the pattern of the modulated light beam which corresponds to the image of the document being printed. Once the light beam scans the photoreceptor for one page, the latent image of one page document being printed is transferred onto the photoreceptor.
The latent image will be made visible by developing it with toner. The toner is generally a colored powder that adheres to the latent image by opposing electrostatic charges. The developed image will then be fixed to a substrate such as plain paper by fusing techniques.
In a color printing system, a full color image is generated by combining four different colors (typically, cyan, yellow, magenta and black). Different printing systems utilize different approaches to generate full color. One approach is to use one raster output scanner and scan one page of document on the photoreceptor once and overscan the same document on the photoreceptor three times for one page of full color document. This approach is called a four pass scanner. Each time the photoreceptor is scanned for one page, all the scan lines of that one page are scanned for one color thus generating one latent image for one color. Also, each time the photoreceptor is overscanned, all the scan lines of that one page will be overscanned to generate one more overlapping latent image. Scanning the photoreceptor once for one document and overscanning it three more times generate a first latent image and three overlapping latent images over the first latent image. Each scan line from each overlapping image has to overlap the same scan line from the first image. By developing each latent image with a different color toner, a full color page of document will be generated.
It should be noted that there are various methods of developing the four colors such as developing the latent image generated on the photoreceptor and then overscanning the photoreceptor for the next color and continue the same procedure for all the colors or generate the four latent images all on a multi-level photoreceptor and then develop each latent image one by one.
Therefore, in the aforementioned approach, a single laser light beam will be utilized to scan the photoreceptor four times for one page.
In a color printing system, it is extremely important to have the four latent images overlap each other with high precision. Each scan line from each latent image must overlap the same scan line from the previous latent images. Misalignment by even a small fraction of a single pixel will produce a color tone error in the copy. This problem is called misregistration.
It is an object of this invention to reduce the misregistration problem.